(1) Field of the Invention
The present invention relates to a secondary cell of the non-aqueous electrolyte type in which lithium is used as the active material for the negative electrode, and manganese oxide, molybdenum trioxide, vanadium pentoxide, titanium sulfide or the like as the active material for the positive electrode.
(2) Description of the Prior Art
This type of cell has the disadvantage of having a very limited period for charge and discharge cycles. It is due to the fact that lithium used as the active material for the negative electrode grows in dendritic forms on surfaces of the negative electrode into contact with the positive electrode to bring about internal short-circuits or becomes deposited in mossy forms and falls from the electrode surfaces.
In order to overcome the above disadvantage, a proposal has been made to form the negative electrode with lithium-aluminum alloy as disclosed in Japanese Patent Publication Kokai No. 52-5423.
Where lithium is used alone as the active material for the negative electrode at the charging time, an elution of lithium ions from the negative electrode render the negative electrode surfaces uneven. At the charging time that follows, lithium becomes deposited in dendritic forms concentratedly on higher positions of the negative electrode surfaces. On the other hand, the use of lithium-aluminum alloy is effective to check such dendritic deposition of lithium since, at the time of charging, lithium is reinstated to form the alloy with aluminum acting as the base of the negative electrode.
However, where the negative electrode is formed of lithium-aluminum alloy using a pure aluminum as the base material of the electrode, the base becomes brittle through repeated charging and discharging. The base will ultimately crumble, thereby deteriorating the cell performance.
Where aluminum alone is alloyed with lithium, the alloy includes .beta.-phase aluminum-lithium alloy formed locally therein which contributes to cell reaction, namely occludes and releases lithium. This .beta.-phase aluminum-lithium alloy is itself brittle, and expands and contracts with the occlusion and release of lithium. As a result, the entire alloy becomes lower in mechanical strength and falls from the negative electrode occur with repeated charging and discharging.
Further, lithium-aluminum alloy causes changes in the electrode configuration with charging and discharging. Therefore, with an increase in the number of repeated charging and discharging, the cell reaction gradually becomes ununiform and lithium becomes unevenly distributed to a greater extent in the electrode. Consequently, the electric potential becomes more negative (i.e. has a higher degree of activity) at locations of high lithium concentration, which readily permits reduction of a solvent acting as an electrolyte.
This unfavorable situation has led to a further proposal, as disclosed in Japanese Patent Publication Kokai No. 61-66369, to add copper or silicon to lithium-aluminum alloy. However, this proposal does not provide satisfactory cell performance for practical purposes.